Recently, high-performance carbon fibers having a high strength and a high modulus of elasticity or molding carbon materials having a high strength and a high modulus of elasticity usable for various purposes by molding under pressure have been demanded eagerly as starting materials for the production of light-weight composite materials having a high strength and a high modulus of elasticity desirable from the viewpoint of saving energy or resources as techniques in aircraft industry, motorcar industry and various other technical fields have progressed.
A process for producing these high-performance carbon materials at low costs by using an optically anisotropic pitch was first disclosed in U.S. Pat. No. 4,005,183. Thereafter, many production processes have been proposed (for example, Japanese Patent Laid-Open Nos. 89635/1975, 118028/1975, 49125/1978 and 55625/1979 and Japanese Patent Publication No. 7533/1978).
However, it has been difficult to produce a homogeneous, optically anisotropic pitch having a low softening point which can be spun stably without using any catalyst on an industrial scale, since these processes have the following defects: (1) the starting material is difficultly available on the market. (2) a long reaction time is required or complicated steps are required, (3) the production cost is high, (4) the softening point of the pitch is raised to make the spinning difficult, and (5) if the softening point of the pitch is controlled, the pitch becomes heterogeneous and, consequently, the spinning becomes difficult. In Japanese Patent Application No. 99646/1980, there is disclosed a process for producing a homogeneous, optically anisotropic pitch having a low softening point on an industrial scale without using any catalyst by overcoming the defects of the conventional techniques. This process for producing an optically anisotropic pitch is characterized in that a heavy oil, tar or pitch mainly comprising heavy hydrocarbons is used as starting material, the starting material is treated at a temperature of above about 380.degree. C. to effect thermal decomposition and polycondensation, the amount of the optically anisotropic phase in the residual pitch is controlled to about 20-80% (percentages of the optically anisotropic pitch herein are given by volume), the polycondensate is stood at a temperature of up to 400.degree. C. to precipitate the optically anisotropic high-density pitch of to form a lower layer in the reaction tank, the pitch is deposited as a growing and aging continuous phase, and the pitch is separated from a part containing a large amount of an optically anisotropic pitch in the upper layer in the reaction tank.
The inventors have noted that the "settling" in the invention of said Japanese Patent Application No. 99646/1980 is not necessarily a completely still state of the reaction mixture but satisfactory results may be obtained if the mixture is not stirred vigorously. After investigations, the inventors have found that the optically anisotropic pitch can be produced not batchwise but continuously on an industrial scale. The present invention has been attained on the basis of this finding.